Numerous speaker systems have been devised to the end of improving the bass or low frequency response of the system, principally in the range of 16 to 100 Hz. Nevertheless, the conventional types of speaker enclosures suffer from various drawbacks. For example, those that employ a flat baffle or panel to which the driver is mounted tend to cause cavity resonance and distortion of the sound. Also such systems require a large baffle area for good low frequency response. In open-backed cabinets in which the driver is mounted on a front panel the speaker effectively has a large diameter-to-length ratio and is inefficient, causes resonant peaks and requires a relatively large cabinet area for a low bass or frequency response. Similarly, a closed cabinet in which the speaker is again mounted on a panel at the rear of the cabinet requires a large diameter-to-length ratio but is nevertheless inefficient, and requires a larger cabinet volume for low frequency response. Nevertheless, it does achieve a flatter response and good damping.
In those cabinets that are ported or provided with a side or front opening so that the speaker is effectively an open tube, the efficiency is improved and requires less cabinet volume for low frequency response.
Labyrinth type enclosures have been proposed in the past in an effort to overcome the deficiencies in low frequency response. For example, U.S. Pat. No. 2,031,500 to Olney couples the back of the speaker cone to the end of a conduit which is folded within a cabinet that places major emphasis on the sound absorbing qualities of the conduit. U.S. Pat. No. 4,628,528 to Bose et al discloses dual folded tubular arrangements between a speaker and opening, the tubes being formed by staggered internal baffles and essentially requires that the pressure wave transmission lines defined by the tubes have an effective length substantially equal to a quarter wave length at the lowest frequency and effectively requires separate transmission lines or passages.
U.S. Pat. No. 4,168,761 to Pappanikolaou is directed more to a labyrinth type speaker enclosure which can maintain free air resonant frequency of a speaker and has a large total internal surface area compared to its volume. It is specifically concerned with providing numerous spaced internal partitions for back waves generated by the speaker and to break them up into substantially annular shape. Another U.S. Letters Patent of interest is that to Olson No. 2,224,919 which discloses the use of dual folded tubes extending from opposite sides of the speaker cone. Pat. Nos. 3,923,124 to Hancock and 4,173,266 to Piser, et al. disclose modified types of folded tube arrangements as do U.S. Pat. Nos. 3,529,691 and 2,646,852 to Wesemann and Forrester respectively.
The present invention overcomes a number of drawbacks and disadvantages in prior art speaker systems in providing for an enclosure containing an audio transmission passage so constructed as to avoid any phase shift in the low frequency range, is extremely compact and readily conformable for use in various sizes and styles of speaker systems and enclosures. The audio transmission passage design of the present invention further lends itself well to compact low profile systems, including portable systems and is further adaptable for use with a bass driver alone or in combination with mid-range and tweeter components. No particular separation or spacing is required between the driver opening and port opening and in general distortion is eliminated even with long excursions.